dear aspirant, as per the CBSE guidelines from the year 2018 onwards one can give improvement exam in those subjects in which the candidate wishes to improve his/her performance. so that they clear the 75% aggregate percentage rule. However as per JOSAA buisness rules either you need to have 75% aggregate or in top 20 percentile of board score.

Earlier it was an exam cundected as a special round to clear the subject or to reappear. Bu as a matter of fact for now this policy haas been changed now improvement in all the subject is mandatory if you bwish to give it for securing better or for clearing its up to you but you need to give in all the respective subjects not for the IIITs or NITs but other as well.